Evangelia A. Pavlatou

E.A. Pavlatou

EUCHEM Conference on Molten Salts (1994), August 21-26, Bad Herrenalb, Germany (oral)

Photon Correlation Spectroscopy (PCS) and spontaneous Brillouin Spectroscopy (BS) have been applied in order to study the dynamic behaviour of structural rearrangements in glassy ZnCl₂ at temperatures below T_g (glass transition)and far above T_m (melting point). The structural properties of the glassy systems ZnCl₂-ZnBr₂ and liquid ZnCl₂-ACl (A=Li, Cs) have been studied by Raman Spectroscopy in the range 25-700°C. The autocorrelation functions of scattered light of pure ZnCl₂ indicate the existence of at least two relaxation processes which occur near T_g. The first "main" process is attributed to local density fluctuations and is very well represented by the KWW equation, exp(-t/τ)^β with β=0.7 insensitive to temperature variations in the region T_g→T_g+80°C. The second "slow" process could be tentatively attributed to "slow" motions of large "clusters" formed in the 3D-network structure. For the melt at high temperatures (BS data) the density relaxation function changes to an exponential form (β=1) and the combination of PCS and BS data revealed a nearly Arrhenius temperature dependence of relaxation time τ through T_g→T_m+200°C. The overall relaxation behaviour categorizes ZnCl₂ in the class of relative "strong"glasses. The temperature dependence of reduced Raman spectra of pure ZnX₂ (X=Cl, Br) indicates the presence of different bonding states in the "network" structure, where halogen atom sharing of the ZnX₄ tetrahedral groups occurs. Furthermore, at elevated temperatures the "network" breaks up creating ZnX₃ units. The spectra of binary ZnCl₂-ZnBr₂ system show that the mixtures are formed by random closest packing of X atoms . In the ZnCl₂-ACl mixtures the breaking of the "network" structure occurs both with increasing temperature and ACl composition.  The tetrahedral structure around Zn is preserved but one, two, three or four "terminal" atoms X are formed in the melt with A⁺ as nearest neighbors.

E.A. Pavlatou

EUCHEM Conference on Molten Salts (1990), September 2-8, Rio, Greece (oral)

Photon Correlation Spectroscopy (PCS) and spontaneous Brillouin Spectroscopy (BS) have been applied to measure the time dependence of the density correlation function g(t) in the mixed glassy slats KNO₃-Ca(NO₃)₂ at different KNO₃ compositions between 50 and 70%. For these mixtures, the density relaxation function changes from a streched exponential (b=0.47) near the glass transition temperature T_g to an exponential (b=1) form for themelt at high temperatures. The relaxation time t depends on the mixture composition and is attributed to the differences between T_g values. In addition, low frequency Raman spectra have been obtained in the glass forming mixtures KNO₃-M(NO₃)₂ M=Ca²⁺, Mg²⁺ at different temperatures and compositions. The reduced depolarized Raman intensity shows a maximum in the low frequency region (<300 cm^-1) which was found to shift (a) to lower energies with increasing temperatures and (b) to higher energies as the NO₃^- counter cation was changed from K⁺ to Ca²⁺ to Mg²⁺. The observed maxima and energy shifts were attributed to reorientational motions of the anisotropic NO₃^- in the glass and the melts.